Combustible gas measurement apparatus and method

ABSTRACT

Discloses a system for the analysis and measurement of selected gases such as combustible gases contained in a gaseous sample to be analyzed including a mixing manifold arrangement to mix the sample gas with a diluting gas to provide a constant gas output flow rate to a detector system. The mixing manifold arrangement automatically reconfigures itself to provide an optimal concentration ratio of sample and diluting mixed gas to the detector system. A constant sample gas input rate is preferred. Excess sample gas not required to maintain a constant mixed sample output rate is discharged.

This is a divisional of co-pending application Ser. No. 09/849,281 filedMay 7, 2001.

FIELD OF THE INVENTION

This invention relates to a system for the analysis and measurement ofselected gases and, more particularly, for measurement of combustiblegases such as hydrocarbon gases contained in a gaseous sample to beanalysed.

BACKGROUND OF THE INVENTION

There is a demand for information indicating the hydrocarbon content ofgaseous mixtures. For example, the return flow drilling mud materialdischarged from an oil or gas exploration well can contain entrainedhydrocarbon gases. Detection and measurement of the hydrocarbon gascontent of the well return material can be used to give an indication ofwhen a certain zone is being penetrated in the well drilling process.Such data can provide information to the geology personnel on thedrilling project to enable them to form an assessment or provide anindication as to whether the well drilling has hit a producing zone. Inoil and gas exploration, the primary hydrocarbon gas of interest isgenerally methane, although, under certain drilling conditions, there isalso interest in information relating to some of the other hydrocarbongases that may be present.

The current state of the art uses a variety of apparatus and methods toquantify and qualify the hydrocarbon content of a gas sample, that is,to perform analysis of the sample. The simplest types of apparatus toperform analysis of a gas sample, are systems that use a “thermalconductivity detector” (TCD). Thermal conductivity detectors aresuitable when the gas to be analyzed by the detector contains a knowngas in a known carrier gas. This is often referred to as binary analysisof gas. Every gas has a unique thermal conductivity as one of itsproperties. Thermal conductivity detection works best when the carriergas and the sample gas have very different thermal conductivities.Typically, the TCD detector has a Wheatstone bridge arrangement wherethe detector element manifests a decrease in resistance with increasingthermal conductivity of the sample gas. By way of example, U.S. Pat. No.3,683,671 to Van Swcray entitled Measuring System Including ThermalConductivity Detector Means provides an electrical circuit bridgeexcited at one power node, by a clamped square wave arm at another powernode by a feedback circuit. The output of the circuit bridge is fed to ademodulator to generate an output signal representative of the samplebeing sensed.

A Wheatstone bridge and visual indicator in the form of light emittingdiodes in a gas analyser arrangement is disclosed in U.S. Pat. No.4,028,057 to Nelson. These detectors are used in gas chromatographywhere a carrier gas that has a very high thermal conductivity, such ashelium, is used. When a sample that has a much lower thermalconductivity than helium is introduced into the carrier gas the outputof the detector will show a change relative to the amount of samplecontained within the carrier. A thermal conductivity detector can beconfused, that is produce erroneous output, if more than one type ofsample gas is introduced into the carrier gas. That is if the thermalconductivity detector is used to analyse a gas mixture of multiplesample gases. For example, if one of the sample gases has a higherthermal conductivity than the carrier gas and the second sample gas hasa lower thermal conductivity than the carrier gas, then the detectoroutput may not even change for varying constituent gas compositions ormixtures.

Thus, a thermal conductivity detector is not well suited to analysis ofhydrocarbon gases entrained in well returns for a number of reasons.First, it is not feasible to transport large tanks full of helium to thewell site. Consequently, the carrier gas that is generally used is air.Air has a thermal conductivity of 1.00 and methane a thermalconductivity of 1.3. This means there is not a very good signal to noiseratio between the air carrier and the gas of interest, which makes athermal conductivity detector based instrument prone to drifting.Notwithstanding their drawbacks, such thermal conductivity detectors arein use in analyzers used in the oil well drilling industry. However,because of the inherent limitations of using TCD detectors in theseenvironments, it is not uncommon to need to zero the baseline of a TCDbased system on an hourly basis. Automated baseline adjustment apparatushave been proposed to compensate for temperature changes in suchsystems. For example, the arrangement proposed by Hagen in U.S. Pat. No.4,817,414.

Also, thermal conductivity detectors are, by their nature, sensitive toambient temperature. Even a 1 degree shift in ambient temperature willcause a noticeable shift in the baseline of a thermal conductivitydetector operating in this low signal to noise ratio configuration.

Another, somewhat more sophisticated detection apparatus employs acatalytic combustion detector (CCD) to detect the presence ofhydrocarbons. For example, U.S. Pat. No. 3,607,084 to Mackey forCombustible Gas Measurement describes passing a stream of gas containingthe combustible gas analytes over a conductive metal wire coated with athink catalytic coating which is at a temperature at which oxidation ofthe gases is initiated. Numerous other arrangements of CCD apparatus areknown for example, U.S. Pat. No. 4,045,177 to McNally, U.S. Pat. No.4,072,467 to Jones, U.S. Pat. No. 4,111,658 to Firth et al, U.S. Pat.No. 4,123,225 to Jones et al, and U.S. Pat. No. 4,313,907 to McNally areexamples of such CCD detectors. CCD's are sensitive to anything that iscombustible and in an oil and gas well drilling environment, hydrocarbongases are the combustible gases that would be encountered. This means aCCD can be used as to provide a measurement of the total hydrocarboncontent of a gas without regard to the particular type of hydrocarbongas. While a CCD will respond to combustible compounds other thanhydrocarbons, it is the gaseous hydrocarbon compounds that will be ofinterest in the sample gases recovered from the drilling mud in a welldrilling environment. A major problem with CCD's is their limited range.If a CCD is subjected to explosive combustible gas concentrations, thatis concentrations between the upper and lower explosive limits of thatcompound, they are destroyed as the gas actually combusts and coats thedetector surface with carbon, rendering it ineffective after that point.For methane the lower explosive limit is 5% in air. An air mixturecontaining methane gas concentrations greater than the 5% lowerexplosive limit will result in a mixture that becomes explosive.

To obtain the benefit of a stable baseline and wider range of methaneconcentrations in a sample, two detector systems have been produced.Current state of the art two-detector apparatus uses a CCD sensor toaround 4% concentration in the mixture. Above that point, the sensorapparatus control turns off the CCD sensor and passes the sensing overto a thermal conductivity sensor. A thermal conductivity sensor, ofcourse, has all of the problems as described above. However, a majoradvantage of a two-detector analyser is a more stable baseline.

A combined CCD and thermal conductivity analyzer has some majordrawbacks if a gas other than methane is present in the sample to beanalyzed. For instance, if C2 is the gas being presented to the CCD, theCCD will detect its presence very nicely. However, when the analyzerswitches over to the thermal conductivity detector, the C2 gas may notbe detected at all. The system will respond by switching back to the CCDwhich ultimately causes the system to keep switching back and forthbetween the two sensors and can result in the destruction of the CCD dueto exposure to explosive levels of C2 gas in the sample. An example of atwo-detector system is shown, for example, in U.S. Pat. No. 4,804,632 toSchuck et al which switches from one sensor to another based on setsample temperatures and holding the sensing devices to a presettemperature.

Another gas detection system using a CCD detector, operates by dilutingthe sample with air when it exceeds 4% as shown, for example, in U.S.Pat. No. 3,771,960 to Kim et al. Adding diluting air to the sampleallows such a gas detection system to use a CCD sensor throughout theentire range. Generally, such gas detection system apparatus providespreset ranges, for example 0% to 3% which is the undiluted range and asecond dilution range, for example 0% to 100%. In one prior artarrangement, the dilution is accomplished by using a manifold withorifices drilled into it that give approximate volumes of gas for thedilution blending. An on/off valve is used to control the diluting ofthe sample with air. This system requires precise adjustment of needlevalves in the factory before being shipped. A problem with this dilutionapproach is that gas concentrations vary considerably with pressure andtemperature and thus are very hard to control precisely enough to givean accurate reading when there is a switch over from one range to theother. In addition to the pressure temperature aspects of the dilutionblending problem, a further problem inherent in this method is that thedilution is very hard to effect without either reducing the sample drawnfrom the extraction device or increasing the amount of sample passedthrough the detector.

In conventional combustible gas analysers, a constant flow rate throughthe detector is maintained by reducing the amount of sample drawn fromthe sample source or extractor. On the other hand, where a constant flowrate from the sample source or extractor is maintained, an increase inthe flow rate through the detector is caused by the air added to orblended with the sample to produce the diluted mixture flowing throughthe detector. Neither of these situations is optimal. Drawing lesssample gas from the gas trap or sample extractor can cause theconcentrations to rise as the gas trap is extracting gas from thedrilling mud at a certain rate. If the rate of sample extraction issuddenly reduced, then there will be a build up of sample gas inside theextractor. On the other hand, if the extraction rate is kept constant,the addition of diluting gas will cause the volume of the diluted samplegas mixture produced to increase with a corresponding increase in thesample flow rate through the detector. Changes in sample flow ratesthrough a CCD detector will consequently change the response of thedetector, as the detector response is dependent on sample flow rates tothe detector. To give accurate results, CCD detectors require a preciseflow rate. In operation, a CCD detector actually destroys sample that itcomes in contact with, so, at low flow rates, the readings will drop offas there is more and more dead sample in contact with the detector.

SUMMARY OF THE INVENTION

To overcome these shortcomings, in one of its aspects, the inventionprovides a sample gas dilution system to control the supply of a samplegas to a detector supply port for supply to a sample detector system.The gas sample dilution system is arranged with three gas flow controls.A sample gas flow control is provided to control input sample gas flowto a detector supply port. A diluting gas flow control is provided tocontrol supply of a diluting gas to the detector supply port andtherefore control blending of the sample gas with the diluting gas. Anexhaust flow control is provided to control an exhaust flow of excesssample gas not required by a sample detector system coupled to thedetector supply port. A controller, such as a computer, provides thesettings of the flow controls. In the preferred manner of operation, thecontroller operates the flow controls to keep the input sample gas flowrate into the sample dilution system constant and the gas flow to thedetector supply port constant. That is, the controller operates thesample gas flow control, the diluting gas flow control, which controlsblending of the sample gas with a diluting gas supply, and the exhaustflow control which controls an exhaust flow of the sample gas tomaintain a constant input sample gas flow rate from the gas samplesource and a constant output flow rate to the detector system. Excesssample gas not required for supply to the sensor block of the sampledetector system is exhausted from the apparatus.

In the preferred embodiment, each gas flow control has a proportionalcontrol valve responsive to a control signal to control the flow of gastherethrough. Preferably closed-loop controlled mass flow controls areutilized to facilitate precise control of gas quantities and flow rates.In a closed-loop controlled gas flow control, the gas flow controlincludes a flow sensor to produce signalling representative of the gasflow rates therethrough. The flow sensor provides a feedback signal thatis used in the control of the proportional control valve to facilitateclosed-loop control of the proportional control valve based on feedbackfrom the flow sensor.

In another aspect of a preferred embodiment of the invention, the sensorblock or detector system operates in conjunction with the sampledilution system to allow for several ranges to be implemented yet keepthe signal to noise ratio from the detector devices at optimum levels.One preferred embodiment discloses ranges of 0% to 4%, 0% to 8%, 0% to16%, 0% to 32%, 0% to 64% and 0% to 100%. An algorithm for automaticrange selection permits optimal sensor block utilisation with minimaluser intervention while providing an output representative ofcombustible gas concentrations in the sample gas without the need toconfigure or reconfigure the instrument manually.

In one of its aspects, the invention provides an apparatus for mixinggases comprising a manifold forming a diluting gas port, a sample inletport and a detector supply port all in common communication with eachother. A diluting gas flow control means is provided which is operableto control a flow of diluting gas through the diluting gas port inresponse to a first control signal. A sample gas flow control means isoperable to control a flow of sample gas to the detector supply port inresponse to a second control signal. A detector means in communicationwith the detector supply port is operable to produce output signallingrepresentative of the content of a selected gas of a gas mixture passingtherethrough. A control means is provided to produce the first andsecond control signals for respective diluting gas and sample gas flowcontrol means whereby any gases supplied to the manifold are mixedtherein and expelled through the detector supply port in proportions setby the control means.

In another of its aspects, the invention provides an apparatus formixing gases comprising a manifold forming a sample gas inlet port, anexhaust port, a diluting gas inlet port and a detector supply port allin common communication with each other. A diluting gas flow controlmeans is operable to control a flow of gas through the diluting gasinlet port in response to a control signal. A sample gas flow controlmeans is provided to control a flow of sample gas to said detectorsupply port in response to a control signal. An exhaust gas flow controlmeans is provided to control a flow of gas through the exhaust port inresponse to a control signal. A control means includes means to receivea detector signal output, the control means produces a respectivecontrol signal for the diluting gas flow control, sample gas flowcontrol and exhaust gas flow control means is included whereby aconstant rate of gas flow through said detector supply port is obtained.The sample gas supplied to the sample gas inlet port and the dilutinggas supplied to the diluting gas inlet port are mixed and expelledthrough the detector supply port in proportions set by the control meansresponsive to a received detector signal output.

And in yet another of its aspects, the invention provides a method ofmeasuring a gas mixture comprising: receiving a sample gas from a sourceat a predetermined sample gas flow rate, supplying a gas mixture to adetector at a predetermined detector supply gas flow rate and receivinga detector signalling produced by a detector monitoring the supplied gasmixture. Periodically the received detector signalling is compared to apredetermined range. A supply of diluting gas is mixed with a selectedportion of the sample gas flow to supply the gas mixture at thepredetermined detector supply gas flow rate and yet maintain thereceived detector signalling within the predetermined range.

Preferred embodiments of the invention will now be described withreference to the attached drawings. For convenience, like referencenumerals have been used to depict like elements of the inventionthroughout the various drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an embodiment of a sampledilution system control in accordance with the invention;

FIG. 2 is a schematic representation of an embodiment of a sampledilution system control of FIG. 1 including mass flow sensors;

FIG. 3 is a flow chart representation of a control process of the sampledilution system of FIG. 1;

FIG. 4 is a schematic representation of another embodiment of a sampledilution system apparatus incorporating features of the inventionwithout an exhaust gas flow control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the discussions contained herein, “flow controller” and “flowcontrol” will refer to any actuator used to regulate the flow by volumeor mass of a selected gas to a predetermined set point. Preferably, theflow control or flow controller has a sensor, that is, either a flowsensor or a mass flow sensor paired with the actuator valve arranged andused in a closed-loop fashion. In the arrangement of the measurementapparatus and method of operation of it, the concepts of mass flow andvolume flow presented herein are used interchangeably. Gases supplied ata constant pressure can provide a constant mass flow. At a constanttemperature and pressure the mass of a gas will be linearly proportionalto its volume, so using mass sensors or flow sensors accomplishes thesame thing. A sensor and actuator valve in a paired arrangement and usedin a closed-loop fashion can be used to regulate the flow of a gas byvolume or by mass. Thus it will be understood that mere rearrangement ofthe relative positions of an actuator valve and flow sensor in a gaspath, or choosing a different control algorithm does not depart from thespirit or scope of the invention as defined in the claims appendedhereto. Thus, in the discussion that follows, “flow” will refer to anyquantity of a selected gas, measured by volume or mass.

FIG. 1 shows, in a schematic diagram form, an embodiment of a sampledilution system of a gas analyzer in accordance with the invention. Thesample dilution system is provided with a source of pressurized cleandry diluting gas, preferably air, for supply to conduit 10. This airpasses through a heater 12 to heat the air to a predetermined uniformtemperature as required, for example, 40° C. The heated air is thenpassed to a regulator 14 to obtain a predetermined uniform air pressure.The temperature and pressure conditioned air is supplied to conduit 16.The gas sample to be measured is supplied to sample tube 18 where it isdelivered to systems to condition the sample to obtain predetermined orpre-set uniform properties. A heater 20 heats the sample to apredetermined temperature, for example, 40° C. A first filter assembly22 removes any particulate matter and airborne liquid or condensedhumidity from the sample. A suitable filter for this purpose is acoalescing filter capable of removing 99.9% of any oil or water dropletsand particulate contamination, preferably to the 0.01 micron level. Apump 24 is used to draw air from the sample source into sample tube 18.

The sample discharged from pump 24 is supplied to conduit 26 andperturbations in pressure caused by operation of pump 24 are absorbed byripple chamber 28. The sample is then fed through a dehumidifier 30 todry the sample to a dew point approaching −40° C. A suitabledehumidifier is a counterflow exchange membrane dryer fabricated fromperfluorinated tetraflouroethylene copolymer membranes, for example,Nafion (trademark) membrane tube counter flow dryers available fromPerma Pure Inc in the MD series gas dryers can be used to dry the sampleair. To operate the dehumidifier 30, a source of dry conditioned air isprovided by air supply conduit 29 that interconnects the dry conditionedair contained in supply line 16 to the dehumidifier 30. The dry airsupplied to dehumidifier 30 passes through an inner chamber or annulus31 of dehumidifier 30 in contact with the exterior surface of the Nafion(trademark) membrane tube 33. The dry air picks up moisture from thesample gas passing though the lumen of membrane 33. The moistureextracted from the gas sample by dehumidifier 30 into the counterflowing dry air flowing in annulus 31 is expelled to the atmosphere bydischarge line 35. The dried sample gas is output from dehumidifier 30into conduit 36.

A liquid filter 32, for example, a micro porous membrane filterconstructed from expanded polytetrafluoroethylene (for example,Teflon*trade mark) is provided as a failsafe to remove any particulatematter that may still be present in the sample stream. A manifold 34,for example a T-junction, forms a sample port 39 to receive the gassample. Manifold 34 communicates the gas received in sample port 39 totwo ports each providing a path for the filtered, dehumidified samplegas to flow along. A first port, namely, a sample exhaust or dischargeport 41, is coupled to a surplus sample discharge line 37 and the otherport, common port 43, is in communication with sample feed or detectorsupply line 38 to deliver the sample gas toward the detector system.Sample gas flows through lines 37 and 38 are controlled by gas flowcontrol means 46 and 48 respectively.

Control of the flow sample gas through discharge line 37 and sample feedline 38 is controlled by gas flow control means, comprising a sampleexhaust or discharge valve 46 and sample valve 48 respectively. Theseflow control means are each responsive to signalling received fromcontroller 44. In this manner, the sample gas passing through samplefeed line 38 is under complete control of controller 44. To provide formore accurate and precise operation and control of the gas flow controlvalves, a closed loop feedback is preferably implemented as will beexplained with reference to FIG. 2. Controller 44 controls the flow ofthe gas sample exiting from manifold 34 through lines 37 and 38respectively. The sample flow rates through lines 37 and 38 are set toprovide for a uniform flow rate of the sample into manifold 34 throughsample port 39 so as to provide a constant sample draw rate from thesample gas source, for example, 500 ml/minute.

The rate of flow of the diluting air, that is the clean dry air, inconduit 16 is controlled by a gas flow control means 52. A gas flowcontrol means 52 is operated in response to signalling from controller44 to control the rate of flow of the clean dry diluting air throughconduit 16. Manifold 54 forms an inlet port 51 coupled to conduit 16carrying the diluting air. Inlet port 51 is in communication with samplesupply port 53 and detector supply port 55, formed by manifold 54. Thesupply of diluting air in conduit 16 and sample gas in conduit 38 arecombined in manifold 54 and then supplied to the sensor block detectormeans 56 via detector supply port 55.

It is preferable that the gas flow into detector sample supply line 58is constant to maintain a constant rate of gas flow into the sensorblock detector means 56. A constant rate of gas flow results in a morereliable and repeatable reading from the sensor apparatus. Controller 44adjusts the mass flow rate, of the diluting air by controlling the dryair gas flow control means valve 52 and of the sample gas by controllingthe sample valve gas flow control means 48, to obtain a uniform massflow rate of the gas mixture into detector sample supply line 58. Forexample, flow control means valves 48, 52 can be controlled to ensurethat a constant flow of gas at the rate of 500 millilitres per minute ofgas is presented to detector sample supply line 58. The gas present indetector sample supply line 58 is heated to a uniform temperature byheater 60, for example, to a temperature of 55° C. The heated sample isthen presented to sensor block detector means 56, which produces anoutput representative of the hydrocarbon gases detected in the sample.The sensor block output is supplied to controller 44 on signal line 62for processing in controller 44. On processing, the controller 44 mayoutput the reading to display 47 for example, or, by supplying thereading in data form on a communications link to a central or a remotecomputer (not shown) for logging and display.

In the preferred embodiment, controller 44 operates to control thesample valve 48 and air valve 52 such that the ratio of sample gas todiluting is mixed at predetermined amounts. For example, a first ratiowhen the sample gas is known to be less than 4% can be used forcalibration. For calibration, a known gas supply, for example 2.5%, isused and fed directly into the gas sample tube 18. In this calibrationconfiguration, controller 44 adjusts the valves of the sample dilutionapparatus to provide 100% of the sample volume and 0% of the dilutingair volume to the sensor block detector means 56. In this calibrationconfiguration, air supply valve 52 and discharge valve 46 are completelyclosed and sample valve 48 is controlled to allow a fixed flow, forexample 500 ml/min. In this configuration of the dilution apparatus,none of the sample gas is exhausted and no diluting air is mixed withthe sample gas before it enters the sensor block detector means 56.

FIG. 2 shows, in a schematic diagram form, a preferred embodiment of thesample dilution system of FIG. 1 that further includes gas flow sensors.

Control of the flow sample gas through discharge line 37 and sample feedline 38 is controlled by gas flow control means, comprising exhaustdischarge valve 46 and sample supply valve 48 respectively. These flowcontrol means are each responsive to signalling received from controller44. In this manner, the sample gas passing through sample feed line 38is under complete control of controller 44. To provide for more accurateand precise operation and control of the gas flow control valves, aclosed loop feedback is preferably implemented. In this regard, adischarge mass flow sensor 40 and a sample mass flow sensor 42, forexample, AWM Series Microbridge Mass Airflow sensors produced byHoneywell, provide an output proportional to the gas mass flow througheach respective mass flow sensor. The outputs of mass flow sensors 40and 42 are used to effect closed-loop control in a control loop. Ifdesired, closed loop control can also be implemented with suitableprocessing in controller 44. Controller 44 sets the mass flow of the gassample exiting from manifold 34 through lines 37 and 38 respectively.The mass flow rates through mass flow sensors 40 and 42 are preferablyselected to provide for a uniform mass flow rate of the sample intomanifold 34 to provide a constant sample draw rate from the sample gassource, for example, 500 ml/minute.

The rate of flow of the diluting air, that is the clean dry air, inconduit 16 is controlled by a gas flow control means 52. Preferably, theflow of air through the actuator valve of gas flow control means 52 ismeasured by a mass flow sensor 50 to obtain the benefit of closed loopcontrol. In one embodiment, controller 44 effects closed loop control,or, in another embodiment, a local feedback loop controller can be usedfor closed loop control. A gas flow control means 52 is operated inresponse to signalling from controller 44 to control the rate of flow ofthe clean dry diluting air through conduit 16. Manifold 54 forms aninlet port 51 coupled to conduit 16 carrying the diluting air. Inletport 51 is in communication with common port 53 and detector supply port55, formed by manifold 54. The supply of diluting air in conduit 16 andsample gas in conduit 38 are combined in manifold 54 and then suppliedto the sensor block detector means 56 via detector supply line 58.

In the preferred manner of operation of the embodiments of the inventiondepicted in FIGS. 1 or 2, controller 44 operates to control the samplevalve 48 and air valve 52 such that the sample air ratio is mixed atpredetermined amounts. For example, a fixed ratio can be configured andused for calibration when the sample gas is known to be less than 4%.For calibration, a known concentration gas supply is used, for example2.5%, and fed directly into the gas sample tube 18. In this calibrationconfiguration, controller 44 adjusts the valves of the sample dilutionapparatus to provide 100% of the sample volume and 0% of the dilutingair volume to the sensor block detector means 56. In this calibrationconfiguration, air supply valve 52 and discharge valve 46 are completelyclosed and sample valve 48 is controlled to maintain a fixed flow, forexample 500 ml/min. In this configuration of the dilution apparatus,none of the sample is exhausted and no diluting air is mixed with thesample before it enters the sensor block detector means 56.

Following is a Dilution Table, which sets out valve configurations thatare set in a preferred method of operating the sample dilutionapparatus. The sample dilution apparatus valve configuration settingsprovide an optimal operating range for supply of sample gas to thesensor block detector means 56. The optimal operating range has an upperthreshold or limit to ensure that the maximum hydrocarbon gasconcentration supplied to the sensor block detector means 56 does notexceed a maximum threshold concentration, for example, a 4%concentration. Also the valve configuration settings of the optimaloperating range provide a lower threshold or limit which increases themixing ratio of sample gas to diluting gas when the predeterminedminimum concentration of sample gas supplied to the sensor blockdetector means 56 falls below the lower threshold. Reducing dilution ofthe sample gas when the detector sensor output falls below apredetermined threshold facilitates obtaining accurate readings from thesensors.

Dilution Table Sample Sample MFS Air MFS Exhaust MFS range % (ml/min) %(ml/min) % (ml/min) 1 <4%   100% (500)    0% (0)    % (0) 2 4-8%   50%(250)    50% (250)    50% (250) 3 8-16%   25% (125)    75% (375)    75%(375) 4 16-32%  12.5% (62.5)  87.5% (437.5)  87.5% (437.5) 5 32-64% 6.25% (31.25)  93.75% (468.75)  93.75% (468.75) 6 64-100 3.125% (15.63)96.875% (484.38) 96.875% (484.38) 7 zeroing 0% (0)   100% (500)   100%(500)

Each row in the table is consecutively numbered and identifies mixingratios and gas mass flows for the particular mixing configuration.Progressively increasing concentrations of the hydrocarbon combustiblegases in the sample are shown row 1 through 6 of the table. Row 7 showsa special zeroing setting that completely closes off sample supply valve48 thereby preventing any sample from entering to the sensor blockdetector means 56.

Because CCD sensor elements may suffer damage or burn out when thehydrocarbon percent gas concentrations are greater than 5%, the sampledilution apparatus is configured at start-up to the maximum dilutionsetting, which is that configuration summarized at row 6 of the DilutionTable. In the configuration of row 6, the gas mixture supplied to thesensor block detector means 56 is diluted to a maximum dilution of thesample and consequently supplies the minimum amount of sample gas to thesensors. In the configuration of row 6, 100% concentrations ofhydrocarbon gas in the sample tube 18 will provide no more than 3.125%concentrations of hydrocarbon gas to the sensor block since 3.125% ofthe sample gas is mixed with 96.875% of the diluting air to provide amaximum mixed gas ratio of 3.125% to the sensor block. Thus in thisconfiguration, a sample gas concentration of 100% hydrocarbon gas willresult in a 3.125% concentration of hydrocarbon gas provided to thesensor block detector means 56.

The hydrocarbon gas concentrations in the sample tube 18 can be relatedto the hydrocarbon gas concentrations provided to the sensor block asfollows:

S=M*X

Where:

X—is the gas concentration of the sample supplied to the sample inlettube 18

M—is the mixing ratio configured, and

S—is the gas concentration provided to the sensor block

FIG. 3 shows, in flow chart form, aspects of the preferred manner ofoperation of the sample dilution apparatus controller 44. Setting themaximum dilution setting shown in row 6 of the Dilution Table isperformed on sample start as depicted by process box 90. In operation ofthe controller 44, when the readings in the sensor block detector means56 fall below a predetermined minimum threshold, for example 1.5%,controller 44 configures the sample dilution apparatus to reduce themixing ratio, that is, to reduce the amount diluting air mixed with thegas sample. With reference to the Dilution Table, reducing the mixingratio moves the mixing configuration up one row, for example, from therow 6 configuration to the row 5 configuration. On the other hand, whenthe mixed sample gas concentration supplied the sensor exceeds themaximum threshold concentration, the next higher mixing ratio isconfigured by the controller 44. With reference to the Dilution Table,increasing the mixing ratio moves the mixing configuration down one row,for example, from the row 1 configuration to the row 2 configuration.

The lower sample concentration limit in the range indicated in theSample Range column of Dilution Table is simply a preferred range anddoes not necessarily cause a reconfiguration to a lower mixing ratiosetting, i.e. moving up a row. The lower range limit is arbitrary and itwill be understood that the ranges can and do overlap. Switching to alower mixing ratio, that is moving up a row in the Dilution table,should not occur unless the gas concentration at the sensor block isless than 1.5% at the time of the reconfiguration. If the mixed gassample concentration at the sensor block detector means 56 is below 2%before a switch to a lower mixing ratio, this avoids providing too richa mixture to the sensor block detector means 56 at the reconfiguredreduced mixing ratio.

On commencement of sample reading, system start-up or after systemreset, the mixing ratio is set to the maximum dilution rate by processbox 90. A sample reading is obtained from the sensors, as shown byprocess box 92 and the reading is output. Each time a reading is output,the output reading takes into account the configuration of the sampledilution apparatus to correct the output amount to the reading obtainedfrom the sensor based on the formula S=M*X referred to previously.Process box 93 represents the output of the reading.

The sample reading obtained is then tested against range limits todetermine if the sample dilution apparatus requires reconfiguring. Atdecision box 94, the sample reading is compared to an upper limit. Ifthe upper limit is exceeded, the “Y” exit is taken and the sampledilution apparatus is reconfigured to increase the dilution amount asrepresented by process box 96 and the next sample is then taken. If theupper limit was not exceeded, then the “N” exit of decision box 94 istaken and the sample reading is then compared to a lower limit atdecision box 98. If the sample reading is below the lower limit, the “Y”exit of decision box is taken and the sample dilution apparatus isreconfigured to decrease the dilution rate as depicted by process box100 and then another sample reading is taken.

To provide a higher degree of control over the hydrocarbon gasconcentrations provided to the sensor block, given that a finite periodof time will be required to reconfigure the sample dilution apparatus(that is, reconfiguration is not instantaneous), the rate of change ofthe sample readings can be monitored as well. At decision box 102 thechange in the current reading to the previous reading is compared to achange limit. If the reading change shows an increase which exceeds anincrease rate limit, the “Y” exit of decision box 102 is taken and thecurrent reading is then evaluated to determined if it is near the uppersensor limit at decision box 104. If the reading is near the upperlimit, the “Y” exit is taken and the sample dilution apparatus isreconfigured to increase the dilution amount as depicted by process box106. This would be equivalent to moving down to the next row inreference to the Dilution Table.

Thus, each sample reading obtained is tested against range limits todetermine if the sample dilution apparatus requires reconfiguring. Whenthe sample gas concentration at the sensor block detector means 56 isbelow the set minimum, controller 44 configures the gas dilutionapparatus to mix less diluting air with the sample. The switchover fromone mixing ratio to the next is controlled in response to the sensorreading data received from the sensor block detector means 56. Thesensor block is thus protected from burnout that would be caused by anypercent gas concentrations greater than 5%. By switching over from onerange to the other when a predetermined threshold, as for example, a1.5% threshold is reached, hysteresis problems that might arise when aswitchover from one range to another are minimized. Controller 44 mayalso include a sample readings derivative or differential factor toswitch from one range to another when readings appear to be rising orfalling quickly so as to ensure that an out of range condition does notoccur in sensor block detector means 56.

FIG. 4 shows another embodiment of a sample dilution systemincorporating features of the invention presented in a schematic diagramform. In this embodiment, no exhaust port is provided in manifold 54.The sample dilution air is supplied under pressure to inlet port 51. Thesample gas to be diluted is supplied to sample port 53. The mixed gasesexit detector supply port 55 for delivery to the sensor block detectormeans 56.

In this embodiment, the gas flow rates must change to effect dilution,and for that reason, this arrangement is not the preferred arrangement.For example, the rate of sample gas flow into sample tube 18 mustdecrease if the rate of mixed gas supply to the sensor block detectormeans 56 is to be constant for all concentrations of hydrocarbons in thesample gas. Or, in another less preferably method of operation, the rateof mixed gas sample flow into sensor block detector means 56 mustincrease if the rate of sample gas flow into sample tube 18 is to remainconstant for all concentrations of hydrocarbons in the sample gas.

The apparatus is arrange such that a gas flow control means 52 isoperated in response to signalling from controller 44 to control therate of flow of the clean dry diluting air through conduit 16. Manifold54 forms an inlet port 51 coupled to conduit 16 carrying the dilutingair. Inlet port 51 is in communication with common port 53 and detectorsupply port 55 formed by manifold 54. The supply of diluting air inconduit 16 and sample gas in conduit 38 are combined in manifold 54 andthen supplied to the sensor block detector means 56.

It is preferable that the gas flow into detector sample supply line 58is constant to maintain a constant rate of gas flow into the sensorblock detector means 56. A constant rate of gas flow results in a morereliable and repeatable reading from the sensor apparatus. Controller 44adjusts the mass flow rate, of the diluting air by controlling the dryair gas flow control means valve 52 and of the sample gas by controllingthe sample valve gas flow control means 48, to obtain a uniform massflow rate of the gas mixture into detector sample supply line 58. Forexample, flow control means valves 48, 52 can be controlled to ensurethat a constant flow of gas at the rate of 500 millilitres per minute ofgas is presented to detector sample supply line 58. Similar to theembodiment described with reference to FIG. 1, the gas present indetector sample supply line 58 is heated to a uniform temperature byheater 60, for example, to a temperature of 55° C. The heated sample isthen presented to sensor block detector means 56, which produces anoutput representative of the hydrocarbon gases detected in the sample.The sensor block output is supplied to controller 44 on signal line 62for processing in controller 44. On processing, the controller 44 mayoutput the reading to display 47 for example, or, by supplying thereading in data form on a communications link to a remote computer (notshown) for logging or display.

In the preferred embodiment, controller 44 operates to control thesample valve 48 and air valve 52 such that the sample air ratio is mixedat predetermined amounts. For example, a first ratio when the sample gasis known to be less than 4% can be used for calibration. Forcalibration, a known 2.5% gas supply is used and fed directly into thegas sample tube 18. In this calibration configuration, controller 44adjusts the valves of the sample dilution apparatus to provide 100% ofthe sample volume and 0% of the diluting air volume to the sensor blockdetector means 56. In this calibration configuration, air supply valve52 and discharge valve 46 are completely closed and sample valve 48 iscompletely open. In this configuration of the dilution apparatus, noneof the sample is exhausted and no diluting air is mixed with the samplebefore it enters the sensor block detector means 56.

Below is a Constant Mixed Gas Sample Output Flow Rate Dilution Table(CMGO Dilution Table), which sets out valve configurations that are setin a preferred method of operating the sample dilution apparatus. Thesample dilution apparatus valve configuration settings provide anoperating range for supply of sample gas to the sensor block detectormeans 56 to ensure that the maximum hydrocarbon gas concentrationsupplied to the sensor block detector means 56 does not exceed a 4%concentration. The valve configuration settings provide a lower range,which ensures that the minimum concentration of sample gas supplied tothe sensor block detector means 56 does not fall below a predeterminedthreshold to facilitate obtaining accurate readings from the sensors.

Constant Mixed Gas Sample Output Flow Rate Dilution Table Sample SampleMFS Air MFS range % (ml/min) % (ml/min) 1 <4%   100% (500)    0% (0) 24-8%   50% (250)    50% (250) 3 8-16%   25% (125)    75% (375) 4 16-32% 12.5% (62.5)  87.5% (437.5) 5 32-64%  6.25% (31.25)  93.75% (468.75) 664-100 3.125% (15.63) 96.875% (464.38) 7 Zeroing   0% (0)   100% (500)

Each row in the table is consecutively numbered and identifies mixingratios and gas mass flows for the particular mixing configuration.Progressively increasing concentrations of the hydrocarbon combustiblegases in the sample are shown row 1 through 6 of the table. Row 7 showsa special zeroing setting that completely closes off sample supply valve48 thereby preventing any sample from entering to the sensor blockdetector means 56.

Because CCD sensor elements may suffer damage or burn out when thehydrocarbon percent gas concentrations are greater than 5%, the sampledilution apparatus is configured at start-up to the maximum dilutionsetting, which is that configuration summarized at row 6 of the CMGODilution Table. In the configuration of row 6, the gas mixture suppliedto the sensor block detector means 56 is diluted to a maximum dilutionof the sample and consequently supplies the minimum amount of sample gasto the sensors. In the configuration of row 6, 100% concentrations ofhydrocarbon gas in the sample tube 18 will provide no more than 3.125%concentrations of hydrocarbon gas to the sensor block as 3.125% of thesample gas is mixed with 96.875% of the diluting air to provide amaximum mixed gas ratio of 3.125% to the sensor block. Thus in thisconfiguration, a sample gas concentration of 100% hydrocarbon gas willresult in a 3.125% concentration of hydrocarbon gas provided to thesensor block detector means 56.

In another manner of operation, the controller 44 operates to controlthe sample valve 48 and air valve 52 such that the sample gasconcentration in the mixer port 55 which supplies the mixed sample gasto the sensor block detector means 56 provides an optimal output, forexample a 2.5% concentration. In this manner of operation, the mixingratio of sample gas to diluting gas is continuously variable. The ratioof sample gas to diluting gas is increased until the desired optimaloutput of the sensor block detector means 56 is obtained. When thedesired optimal output is obtained, the mixing ratio of the sample gasto the diluting gas is known, and, consequently, the concentration ofthe sample gas is determined. Naturally the sample gas concentration maybe below the concentration necessary to produce the optimal output ofthe detector sensor, in which case, the output of the detector sensorwill be correspondingly reduced.

Now that the preferred embodiments of the invention have been describednumerous changes and modifications may be made thereto without departingfrom the spirit and scope of the invention as defined in the claimsappended hereto.

We claim:
 1. A method of measuring a gas mixture comprising: (i).receiving a sample gas from a source at a predetermined sample gas flowrate; (ii). supplying a gas mixture to a detector at a predetermineddetector supply gas flow rate; (iii) receiving a detector signallingproduced by a detector monitoring the gas mixture supplied by step (ii);(iv) periodically comparing the received detector signalling to apredetermined range; (v) mixing a supply of diluting gas with a portionof the sample gas flow to supply the gas mixture at the predetermineddetector supply gas flow rate of step (ii) to maintain the receiveddetector signalling within the predetermined range; whereby the samplegas received in step (i) is received at a predetermined sample gas flowrate and the gas mixture supplied to the detector is supplied at apredetermined detector supply gas flow rate.
 2. The method of claim 1further wherein the predetermined range has an upper threshold.
 3. Themethod of claim 2 wherein the upper threshold is 4 percent.
 4. Themethod of claim 2 wherein the gas mixture supplied to the detector issupplied at a gas flow rate obtained by mixing a supply of diluting gaswith a portion of the sample gas flow to supply the gas mixture at apredetermined amount selected from one of: (1) 100 percent sample gasmixed with 0 percent diluting gas; (2) 50 percent sample gas mixed with50 percent diluting gas; (3) 25 percent sample gas mixed with 75 percentdiluting gas; (4) 12.5 percent sample gas mixed with 87.5 percentdiluting gas; (5) 6.25 percent sample gas mixed with 93.75 percentdiluting gas; (6) 3.125 percent sample gas mixed with 96.875 percentdiluting gas; and (7) 0 percent sample gas mixed with 100 percentdiluting gas.
 5. The method of claim 2 further including the steps of:(i) periodically comparing the received detector signalling to the upperthreshold and when the received detector signalling exceeds the upperthreshold: (i.1) reducing the portion of the sample gas flow mixed withthe supply of diluting gas when the periodic comparison of the receiveddetector signalling is above a predetermined, and (i.2) increasing thesupply of diluting gas; whereby continuous receipt of sample gas isobtained at a predetermined sample gas flow rate.
 6. The method of claim1 further wherein the predetermined range has a lower threshold.
 7. Themethod of claim 6 wherein the lower threshold is 1.5 percent.
 8. Themethod of claim 6 further including the steps of: (i) periodicallycomparing the received detector signalling to the lower threshold andwhen the received detector signalling is below the lower threshold:(i.1) increasing the portion of the sample gas flow mixed with thesupply of diluting gas when the periodic comparison of the receiveddetector signalling is above a predetermined, and (i.2) decreasing thesupply of diluting gas, whereby continuous receipt of sample gas isobtained at a predetermined sample gas flow rate.
 9. The method of claim6 wherein the gas mixture supplied to the detector is supplied at a gasflow rate obtained by mixing a supply of diluting gas with a portion ofthe sample gas flow to supply the gas mixture at a predetermined amountselected from one of: (1) 100 percent sample gas mixed with 0 percentdiluting gas; (2) 50 percent sample gas mixed with 50 percent dilutinggas; (3) 25 percent sample gas mixed with 75 percent diluting gas; (4)12.5 percent sample gas mixed with 87.5 percent diluting gas; (5) 6.25percent sample gas mixed with 93.75 percent diluting gas; (6) 3.125percent sample gas mixed with 96.875 percent diluting gas; and (7) 0percent sample gas mixed with 100 percent diluting gas.